A coil for such machinery comprises a loop of copper (or other electrical conductor) strip or rectangular section rod or wire having perhaps four turns having a primary insulation from each other. The loop is spread so that there are two equal, parallel sections connected by an "overhang" at each end, the ends of the strip or rod coming out at one end of the coil and connected to terminals, for example, of a commutator. The two parallel sections, the cross-sections of which are usually relatively inclined, are intended to fit into slots in the rotor or stator of the machine. Their relative inclination means that the overhangs take on a somewhat complicated, kinked shape on spreading the loop.
Before insertion into the slots, the coils require further insulation, which is applied in one of a variety of ways depending upon the operating voltage. For high voltage machines, the straight, parallel sections of the coils are covered with flake mica, whereas for lower voltage machines the mica is not required. In any event, the outermost layer or layers of insulation comprise a wrapped tape material.
Conventionally, the tape is a paper tape, and, for high temperature use, the tape is often made from a high temperature synthetic fibre such as that marketed by E. I. du Pont de Nemours under the Registered Trade Mark "Nomex".
The high temperature nylon paper tape, however, has certain disadvantages in practise. For one thing, it is quite stiff and cannot be wound very satisfactorily around the corners and kinks of the coil. To improve its flexibility, it is available in crimped or corrugated form, but even this is not a complete answer. Moreover, the tape is not isotropic, and can tear quite easily in the cross direction.
It is required, also, to impregnate the insulation with resin, which is then cured, to fill any air gaps in the insulation and provide a solid and coherent coating which is at the same time capable of good heat transfer and good electrical insulating properties. The Nomex paper tape does not absorb the resin very well.
A further disadvantage of the Nomex paper tape is that it is very expensive.